Method of cooling a wafer support at a uniform temperature in a capacitively coupled plasma reactor

ABSTRACT

A method of transferring heat from or to a workpiece support in an RF coupled plasma reactor includes placing coolant in an internal flow channel that is located inside the workpiece support and transferring heat from or to the coolant by circulating the coolant through a refrigeration loop in which the internal flow channel of the workpiece support constitutes an evaporator of the refrigeration loop. The method further includes maintaining thermal conditions of the coolant inside the evaporator within a range in which heat exchange between the workpiece support and the coolant is primarily or exclusively through the latent heat of vaporization of the coolant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/410,782 filed Apr. 24, 2006 entitled A METHOD OF COOLING A WAFERSUPPORT AT A UNIFORM TEMPERATURE IN A CAPACITIVELY COUPLED PLASMAREACTOR by Paul Brillhart, et al., which claims the benefit of U.S.Provisional Patent Application Ser. No. 60/725,763, filed Oct. 11, 2005.All of the above applications are hereby incorporated by reference intheir entirety.

BACKGROUND OF THE INVENTION

In a capacitively coupled plasma reactor, control over dissociation hasbeen achieved with a wide impedance match space at very high RF sourcepower over a very wide chamber pressure range. Such a wide operatingrange is attributable, at least in part, to a unique feature of theoverhead electrode matched to the RF power source by a fixed impedancematching stub with the following features. First, the electrodecapacitance is matched to the plasma reactance at a plasma-electroderesonant frequency. The stub resonant frequency, the plasma-electroderesonant frequency and the source frequency are nearly matched at a VHFfrequency. A highly uniform etch rate across the wafer is attainedthrough a number of features. These features include, among otherthings, the adjustment of the bias power feedpoint impedance on theelectrostatic chuck to provide a radially uniform RF impedance acrossthe chuck for both its role as an RF bias power applicator and as an RFreturn for the VHF source power from the overhead electrode. Thisadjustment is made by dielectric sleeves around the bias feed line ofuniquely selected dielectric constants and lengths. Another feature is adielectric ring process kit for the cathode periphery to combat edgeeffects. Other features that can further improve process or etch ratedistribution uniformity include dual zone gas feeding, curving of theoverhead electrode and plasma steering magnetic fields. A plasma reactorthat includes many of these key features provides an etch ratedistribution uniformity that surpasses the conventional art.

With rapid shrinking of circuit feature sizes, the requirements for etchrate distribution uniformity are so stringent that small temperaturevariations across the wafer must now be minimized or eliminated, withthe added proviso that future sophisticated process recipes designed tomeet the latest stringent requirements will require agile and highlyaccurate time-changing wafer temperature profiling, and/or RF heat loadprofiling. Such changes must be effected or compensated with thegreatest temperature uniformity across the wafer. How to do all thiswithout degrading the now highly uniform etch rate distributioncurrently afforded by the reactor is a difficult problem. Moreover, suchhighly accurate and agile temperature control or profiling requiresaccurate temperature sensing at the wafer. However, introduction oftemperature probes near the wafer will create parasitic RF fields whichdistort the fine effects of the feed-point impedance dielectric sleevesand the dielectric ring process kit, defeating their purpose.Temperature non-uniformities at the wafer arising from lack of control,to the extent that they impact the etch chemistry, will have the sameultimate effect of distorting an otherwise uniform environment.

Conventional cooling systems for regulating the temperature of the wafersupport pedestal or electrostatic chuck employ a refrigeration systemthat cools a refrigerant or coolant medium using a conventional thermalcycle and transfers heat between the coolant and the electrostatic chuckthrough a separate liquid heat transfer medium. The coolant may be amixture of deionized water with other substances such as glycol and (or)perfluoropolyethers. One problem with such systems is that, at high RFpower levels (high RF bias power or high RF source power or both), suchcooling systems allow the wafer temperature to drift (increase) for asignificant period before stabilizing after the onset of RF power. Suchtemperature drift has two phases. In a brief initial phase, theelectrostatic chuck is at an ambient (cold) temperature when RF power isfirst applied, so that the temperature of the first wafer to beintroduced climbs rapidly toward equilibrium as the RF heat load slowlyheats the chuck. This is undesirable because the wafer temperature risesuncontrollably during processing. Even after the electrostatic chuck(ESC) has been heated by the RF heat load, the wafer temperature driftsupwardly and slowly approaches an equilibrium temperature. Such driftrepresents a lack of control over wafer temperature, and degrades theprocess. The drift is caused by the inefficiency of the conventionalcooling process.

Another problem is that rapid temperature variations between twotemperature levels cannot be carried out for two reasons. First, theheat transfer fluid that provides thermal transfer between the ESC andthe coolant has a heat propagation time that introduces a significantdelay between the time a temperature change is initiated in therefrigeration loop and the time that the wafer actually experiences thetemperature change. Secondly, there is a heat propagation time delaybetween the cooled portion of the ESC base and the wafer at the top ofthe ESC, this time delay being determined by the mass and heat capacityof the materials in the ESC.

One of the most difficult problems is that under high RF heat load onthe wafer requiring high rates of thermal transfer through the cooledESC, the thermal transfer fluid temperature changes significantly as itflows through the fluid passages within the ESC, so that temperaturedistribution across the ESC (and therefore across the wafer) becomesnon-uniform. Such non-uniformities have not presented a significantproblem under older design rules (larger semiconductor circuit featuresizes) because etch rate uniformity across the wafer diameter was not ascritical at the earlier (larger) feature sizes/design rules. However,the current feature sizes have dictated the extremely uniform electricfields across the ESC achieved by the features described above (e.g., RFbias feedpoint impedance adjustment, process kit dielectric edge rings).However, the high RF heat loads, dictated by some of the latest plasmaetch process recipes, cause temperature non-uniformities across thewafer diameter (due to sensible heating of the thermal transfer fluidwithin the ESC) that distort an otherwise uniform etch rate distributionacross the wafer. It has seemed that this problem cannot be avoidedwithout limiting the RF power applied to the wafer. However, as etchrate uniformity requirements become more stringent in the future,further reduction in RF power limits to satisfy such requirements willproduce more anemic process results, which will ultimately beunacceptable. Therefore, there is a need for a way of extracting heatfrom the wafer under high RF heat load conditions without introducingtemperature non-uniformities across the ESC or across the wafer.

SUMMARY OF THE INVENTION

A method of transferring heat from or to a workpiece support in an RFcoupled plasma reactor includes placing coolant in an internal flowchannel that is located inside the workpiece support and transferringheat from or to the coolant by circulating the coolant through arefrigeration loop in which the internal flow channel of the workpiecesupport constitutes an evaporator of the refrigeration loop. The methodfurther includes maintaining thermal conditions of the coolant insidethe evaporator within a range in which heat exchange between theworkpiece support and the coolant is primarily or exclusively throughthe latent heat of vaporization of the coolant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a capacitively coupled plasma reactor embodyingfeatures of the invention.

FIG. 2 is a schematic diagram of the RF bias power feed circuit of thereactor of FIG. 1.

FIG. 3 is a top view corresponding to FIG. 2.

FIG. 4 is a detailed diagram of a coaxial feed portion of the circuit ofFIG. 2.

FIG. 5 illustrates a first dielectric ring process kit in the reactor ofFIG. 1.

FIG. 6 illustrates a second dielectric ring process kit in the reactorof FIG. 1.

FIG. 7 illustrates a system including the reactor of FIG. 1 embodyingthe invention.

FIG. 8 is a graph of the temperature as a function of enthalpy of thecoolant inside the evaporator of FIG. 7, and further depicting thedome-shaped liquid-vapor phase boundary.

FIG. 9 is a block flow diagram of a two-phase constant temperaturecooling process of the invention.

FIG. 10 depicts an exemplary wafer temperature-time profile that may berealized using the invention.

FIGS. 11A and 11B are contemporary timing diagrams of the wafertemperature and wafer backside gas pressure, respectively, in accordancewith a process for stepping the wafer temperature down in advance of acorresponding ESC temperature change.

FIGS. 12A and 12B are contemporary timing diagrams of the wafertemperature and wafer backside gas pressure, respectively, in accordancewith a process for stepping the wafer temperature down after completionof a corresponding ESC temperature change.

FIG. 13 illustrates a system similar to that of FIG. 7 but havingmultiple temperature control loops governing respectively multipletemperature zones.

FIG. 14 illustrates an optical temperature sensor of the invention asinstalled in the ESC of FIG. 7 or FIG. 13.

FIG. 15 illustrates an upper probe of the temperature sensor of FIG. 14.

FIG. 16 illustrates a lower probe of the temperature sensor of FIG. 14.

FIG. 17 is an enlarged view of a portion of FIG. 14 showing how theupper and lower probes are joined together within the ESC.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a plasma reactor includes a reactor chamber 100with a wafer support 105 at the bottom of the chamber supporting asemiconductor wafer 110. A semiconductor ring 115 surrounds the wafer110. The semiconductor ring 115 is supported on the grounded chamberbody 127 by a dielectric (quartz) ring 120. The chamber 100 is boundedat the top by a disc shaped overhead electrode 125 supported at apredetermined gap length above the wafer 110 on grounded chamber body127 by a dielectric (quartz) seal 130. An RF generator 150 applies RFplasma source power to the electrode 125. RF power from the generator150 is coupled through a coaxial cable 162 matched to the generator 150and into a coaxial stub 135 connected to the electrode 125. The stub 135has a characteristic impedance, resonant frequency determined by itslength, and provides an impedance match between the electrode 125 andthe 50 Ohm coaxial cable 162 or the 50 Ohm output of the RF powergenerator 150. The chamber body is connected to the RF return (RFground) of the RF generator 150. The RF path from the overhead electrode125 to RF ground is affected by the capacitance of the semiconductorring 115, the dielectric ring 120 and the dielectric seal 130. The wafersupport 105, the wafer 110 and the semiconductor ring 115 provide theprimary RF return path for RF power applied to the electrode 125.

A large impedance match space is realized when the source powerfrequency, the plasma electrode resonance frequency and the stubresonance frequency are nearly matched. Preferably, three frequenciesare slightly offset from one another, with the source power frequencybeing 162 MHz (optimized for 300 mm wafers), the electrode-plasmaresonant frequency being slightly below 162 MHz, and the stub resonancefrequency being slightly above 162 MHz, in order to achieve a de-tuningeffect which advantageously reduces the system Q. Such a reduction insystem Q renders the reactor performance less susceptible to changes inconditions inside the chamber, so that the entire process is much morestable and can be carried out over a far wider process window.

The electrode capacitance is matched to the magnitude of the negativecapacitance of the plasma, and the resulting electrode-plasma resonantfrequency and the source power frequency are at least nearly matched.For the typical metal and dielectric etch process conditions (i.e.,plasma density between 10⁹-10¹² ions/cc, a 2-inch gap and an electrodediameter on the order of roughly 12 inches), the match is possible ifthe source power frequency is a VHF frequency.

An advantage of choosing the capacitance of the electrode 125 in thismanner, and then matching the resultant electrode-plasma resonantfrequency and the source power frequency, is that resonance of theelectrode and plasma near the source power frequency provides a widerimpedance match and wider process window, and consequently much greaterimmunity to changes in process conditions, and therefore greaterperformance stability. Matching the stub resonance frequency to theelectrode plasma resonant frequency minimizes reflections at thestub-electrode interface. The entire processing system is rendered lesssensitive to variations in operating conditions, e.g., shifts in plasmaimpedance, and therefore more reliable along with a greater range ofprocess applicability.

In accordance with a further aspect, the system Q is reduced to broadenthe process window by slightly offsetting the stub resonant frequency,the electrode plasma resonant frequency and the plasma source powerfrequency from one another. The use of the higher VHF source powerfrequency proportionately decreases the Q as well. Decreasing system Qbroadens the impedance match space of the system, so that itsperformance is not as susceptible to changes in plasma conditions ordeviations from manufacturing tolerances.

Bias Circuit Tuning for Uniform Radial Plasma Distribution:

Continuing to refer to FIG. 1, the workpiece support cathode 105includes a metal base layer 05 supporting a lower insulation layer 10,an electrically conductive mesh layer 15 overlying the lower insulationlayer 10 and a thin top insulation layer 20 covering the conductive meshlayer 15. The semiconductor workpiece or wafer 110 is placed on top ofthe top insulation layer 20. RF bias power is coupled to the conductivemesh layer 15 to control ion bombardment energy at the surface of thewafer 110. The conductive mesh 15 also can be used for electrostaticallychucking and de-chucking the wafer 110, and in such a case can beconnected to a chucking voltage source in the well-known fashion. Theconductive mesh 15 therefore is not necessarily grounded and can have,alternately, a floating electric potential or a fixed D.C. potential inaccordance with conventional chucking and de-chucking operations. Themetal base layer 05 typically (but not necessarily) is connected toground, and forms part of a return path for VHF power radiated by theoverhead electrode 125.

An RF bias generator 40 produces power in the HF band (e.g., 13.56 MHz).Its RF bias impedance match element 45 is coupled to the conductive mesh15 by an elongate conductor 25 (hereinafter referred to as an RFconductor) extending through the workpiece support cathode 105. The RFconductor 25 is insulated from grounded conductors such as the aluminumbase layer 05. The RF conductor 25 has a top termination or bias powerfeed point 25 a in electrical contact with the conductive mesh 15.

FIG. 2 is a schematic illustration corresponding to FIG. 1 of thecircuit consisting of the VHF overhead electrode 125, the RF biasapplied through the workpiece support cathode 105 and the elements ofthe cathode 105. FIG. 3 is a top plan view corresponding to FIG. 1 ofthe plane of the wafer 110, with the termination or feed point 25 a ofthe RF conductor 25 being shown in hidden (dashed) line. The RF returnpath provided by the workpiece support cathode 105 consists of twoportions in the plane of the wafer 110, namely a radially inner portion30 centered about and extending outwardly from the feed point 25 a and aradially outer annular portion 35. The RF return paths provided by thetwo portions 30, 35 are different, and therefore the two portions 30, 35present different impedances to the VHF power radiated by the overheadelectrode 125. Such differences may cause non-uniformities in radialdistribution across the wafer surface of impedance to the VHF power,giving rise to nonuniform radial distribution of plasma ion density nearthe surface of the workpiece.

In order to solve this problem, a dielectric cylindrical sleeve 50(shown in the enlarged view of FIG. 2) surrounds the RF conductor 25.The axial length and the dielectric constant of the materialconstituting the sleeve 50 determine the feed point impedance presentedby the RF conductor 25 to the VHF power. In one example, the length anddielectric constant of the sleeve 50 is selected to bring the feed pointimpedance to nearly zero at the VHF source power frequency (e.g., 162MHz). The impedance presented by the outer region 35 surrounding thefeed point 25 a is nearly a short at 162 MHz (due mainly to the presenceof the conductive mesh 15). Therefore, in the latter example the sleeve50 may bring the feed point impedance at the source power frequency to avalue closer to that of the surrounding region. Here, the impedance ofthe region surrounding the feed point is determined mainly by theconductive mesh 15. As a result, a more uniform radial distribution ofimpedance is attained, for more uniform capacitive coupling of VHFsource power.

The sleeve 50 can include additional features facilitating theforegoing, improvement in VHF power deposition while simultaneouslysolving a separate problem, namely improving the uniformity in theelectric field created by the RF bias power (at 13.56 MHz for example)applied to the wafer 110 by the RF conductor 25. The problem is how toadjust radial distribution of VHF power coupling for maximum uniformityof plasma ion density while simultaneously adjusting the HF bias powerelectric field distribution across the wafer surface for maximumuniformity.

FIG. 4 is an enlarged view corresponding to FIGS. 1-3 showing how thesleeve 50 can be divided into three sections, namely a top section 52, amiddle section 54 and a bottom section 56. The length and dielectricconstant of the sleeve top section 52 is selected and fixed to optimizethe HF bias power deposition exclusively, and the lengths and dielectricconstants of the remaining sleeve sections 54, 56 are then selected tooptimize VHF source power deposition by the overhead electrode whileleaving the HF bias power deposition optimized.

RF Coupling Ring for Enhancing Plasma Uniformity:

Center-high plasma distribution non-uniformity is reduced by selectivelyenhancing capacitive coupling from the overhead electrode 125 to theplasma in the vicinity of the workpiece periphery. FIG. 5 corresponds toan enlarged view of FIG. 1 illustrating the additional feature of anannular RF coupling ring that is placed over and in electrical contactwith the outer periphery of the wafer support cathode 105. As shown inFIG. 5, the top insulation layer 20 is surrounded by a removable ring 80whose top surface 80 a is coplanar with the top surface of the wafer110. The removable ring 80 can be formed of a process-compatiblematerial such as silicon, for example. Optionally, removable metalground ring 85 surrounds the removable ring 80, its top surface 85 abeing coplanar with that of the removable ring 80. A generally planarsurface is provided across the top of the wafer support cathode 105bounded by the periphery of the ground ring 85, facing the generallyplanar surface of the bottom of the overhead electrode 125. As a result,capacitive coupling across the entire processing zone bounded by theoverhead electrode 125 and the wafer support cathode 105 is generallyuniform. In order to overcome non-uniformity inherent in the center-highplasma ion density distribution of the reactor, capacitive coupling bythe overhead electrode 125 is enhanced near the outer portion of theworkpiece 110 by placing an RE coupling ring 90 over the removable ring80 and over grounded ring 85. The RF coupling ring 90 may be aconductor, a semiconductor or a dielectric. If the coupling ring 90 is adielectric, then capacitive coupling to the plasma near the waferperiphery is enhanced by the presence of the dielectric material. If theRF coupling ring 90 is a conductor, it in effect narrows theelectrode-to-counterelectrode spacing and thereby enhances capacitancenear the peripheral region of the wafer 110. Thus, theelectrode-to-counterelectrode spacing is h1 everywhere in the processzone except at the periphery occupied by the RF coupling ring 90 wherethe spacing is reduced from h1 by the height h2 of the coupling ring 90.The increased capacitive coupling of source power enhances ion densityat the periphery. The increase in ion density extends inwardly from theRF coupling ring 90 and extends over a peripheral portion of theworkpiece 110. Thus, the plasma ion density over the workpiece 110 isless center high and may tend toward being more nearly uniform, orpossibly slightly edge-high. This condition is optimized by a carefulselection of the height (thickness) h2 of the RF coupling ring 90.

FIG. 6 illustrates a modification of the reactor of FIG. 5 in which asecond RF coupling ceiling ring 95 is attached to the periphery of thebottom surface of the overhead electrode 125 and overlies the first RFcoupling ring 90. If each ring 90, 95 has a thickness (height) of h3,then the electrode-to-counterelectrode distance near the wafer peripheryis reduced by twice h3 and the capacitance in that region is enhancedproportionately, as in the reactor of FIG. 5.

With the RF coupling ring 90 and the dielectric sleeve 50, plasma iondensity distribution uniformity is improved. Any remainingnon-uniformities can be corrected by plasma-steering magnetic fieldscontrolled by a plasma distribution controller 57 (shown in FIG. 1)governing D.C. current sources 58, 59 that drive overhead coils 60, 65.

Another modification that can be employed to enhance plasma processinguniformity across the diameter of the wafer 110 is to change the planarelectrode surface 125 a to a convex curved electrode surface 125 b. Thedegree of curvature can be selected to compensate for non-uniform plasmaion density radial distribution that may exist with the planar electrodesurface 125 a.

Highly Efficient Temperature Control Apparatus:

FIG. 7 is an enlarged view of the wafer support pedestal 105 of FIG. 1,revealing the internal structure of the pedestal 105. The pedestal 105embodies an electrostatic chuck (ESC), as described in FIG. 2, FIG. 7showing that the aluminum base 5 contains flow passages 200 for a PCHTmedium with an inlet 201 and an outlet 202. The internal flow passages200 constitute the heat exchanger of a PCHT loop, the heat exchanger 200being internally contained with the ESC base 5. The PCHT loop canoperate in either of two modes, namely a cooling mode (in which the heatexchanger 200 functions as an evaporator) and a heating mode (in whichthe heat exchanger 200 functions as a condenser). The remaining elementsof the PCHT loop are external of the ESC 105, and include (in order ofPCHT medium flow direction, starting from the outlet 202) an accumulator204, a compressor 206 (for pumping the PCHT medium through the loop),and (for the cooling mode of operation) a condenser 208 and an expansionvalve 210 having a variable orifice size, all of which are of the typewell-known in the art. An advantage of locating the heat exchanger 200inside the ESC base 05 is that the delay and losses inherent in thethermal transfer fluid of the prior art are eliminated. The PCHT loop(i.e., the heat exchanger 200, the accumulator 204, the compressor 206,the condenser 208, the expansion valve 210 and the conduits couplingthem together, contain the PCHT medium (which functions as a refrigerantor coolant when the PCHT operates in the cooling mode) of a conventionaltype and can have low electrical conductivity to avoid interfering withthe RF characteristics of the reactor. The accumulator 204 prevents anyliquid form of the PCHT medium from reaching the compressor 206 bystoring the liquid. This liquid is converted to vapor by appropriatelyoperating the bypass valve 214.

In order to overcome the problem of thermal drift during processing, theefficiency of the PCHT loop is increased ten-fold or more by operatingthe PCHT loop 200, 204, 206, 208, 210 so that the PCHT medium inside theheat exchanger is divided between a liquid phase and a vapor phase. Theliquid-to-vapor ratio at the inlet 201 is sufficiently high to allow fora decrease in this ratio at the outlet 202. This guarantees that all (ornearly all) heat transfer between the ESC base 05 and the PCHT medium(coolant) within the heat exchanger (evaporator) 200 occurs throughcontribution to the latent heat of evaporation of the PCHT medium. As aresult, the heat flow in the PCHT loop exceeds, by a factor of 10, theheat flow in a single-phase cooling cycle. This condition can besatisfied with a decrease in the CPHT medium's liquid-to-vapor ratiofrom the inlet 201 to the outlet 202 that is sufficiently limited sothat at least a very small amount of liquid remains at (or just before)the outlet 202. In the cooling mode, this requires that the coolantcapacity of the PCHT loop is not exceeded by the RF heat load on thewafer. One way of ensuring this is to provide the PCHT loop with amaximum cooling capacity that is about twice the maximum anticipatedheat load on the wafer. In one implementation of a reactor of the typedepicted in FIGS. 1-7, the maximum cooling rate of the PCHT loop wasbetween about three and four times the maximum anticipated heat load onthe wafer. The heat load on the wafer was about 30% of the applied RFpower on the wafer. The liquid-to-vapor ratio was between about 40% and60% at the inlet 201 and about 10% at the outlet 202.

While the PCHT loop has been described with reference primarily to thecooling mode of operation, it can also be employed in a heating modewhenever it is desired to raise the temperature of the ESC (e.g., at afaster rate than plasma heating alone is capable of). For operation ofthe PCHT loop in the heating mode, the condenser 206 and expansion valve210 are bypassed by at least some of the PCHT medium by opening thebypass valve 212, so as to allow superheated PCHT medium to flow to theheat exchanger 200. In this case, the heat exchanger 200 functions as acondenser rather than an evaporator. In this mode (the heating mode),overheating of the compressor 206 may be prevented by providing anadditional bypass (not shown) from the output of the condenser 206 tothe input of the compressor 208. In the heating mode, theliquid-to-vapor ratio in the heat exchanger 200 may be zero.

FIG. 8 is a phase diagram depicting the enthalpy of the PCHT mediuminside the heat exchanger 200 as a function of temperature. Thetemperature-enthalpy boundary between the three phases (liquid, solid,vapor) is a liquid-vapor dome 216 beneath which the PCHT medium existsin both liquid and vapor phases. To the lower enthalpy side of the dome216, the PCHT medium is a sub-cooled (100%) liquid phase while to thehigher enthalpy side of the dome 216 the PCHT medium is a superheated(100%) vapor. At the apex of the dome is the triple point at which allthree phases of the PCHT medium are present simultaneously. Thecontrollable parameters of the PCHT loop of FIG. 7, (i.e., the PCHTmedium flow rate established by the compressor 206, the orifice size ofthe expansion valve 210 and the opening size of a bypass valve 212 thatwill be discussed later herein) are selected by the skilled worker sothat the temperature and enthalpy of the PCHT medium inside the heatexchanger 200 stays under or within the liquid-vapor dome 216 of thephase diagram of FIG. 8. The pressure inside the heat exchanger 200 ismaintained at a constant level provided that a constant ESC basetemperature is desired, so that there is theoretically no temperaturechange as the coolant flows through the heat exchanger 200, as indicatedby the perfectly horizontal lines of constant pressure 218 a, 218 b ofFIG. 8. (In actual practice, there is a negligible temperaturedifference across the ESC inlet and outlet 201, 202 of about 5 degreesC. or less under typical operating conditions.) As the PCHT mediuminside the evaporator 200 absorbs heat from the ESC base 5, its internalenergy U increases, causing its enthalpy to increase (where enthalpy isU+PV, P and V being pressure and volume inside the evaporator 200). Tosatisfy the requirement for two-phase heat transfer through latent heatof evaporation exclusively (or nearly exclusively) as defined above, thePCHT medium's enthalpy/temperature coordinates must remain inside theliquid-vapor dome 216 of FIG. 8. Thus, for a constant pressure, the PCHTmedium's temperature/enthalpy coordinates follow a line of constantpressure (e.g., line 218 a) entering the heat exchanger 200 at a lowenthalpy (labeled “inlet” in FIG. 8) and exiting at a higher enthalpy(labeled “outlet” in FIG. 8), with the entry and exit enthalpies lyinginside or on the boundary of the liquid-vapor dome 216. FIG. 8 showsthat a greater increase in enthalpy (absorbed heat) is achieved at lowercoolant temperatures.

Solution to the Problem of Non-Uniform Temperatures Across the Esc andWafer:

Maintaining the PCHT medium (hereinafter referred to as “coolant”)inside the evaporator 200 of FIG. 7 within the liquid-vapor dome of FIG.8—to guarantee heat extraction through the latent heat of vaporizationalmost exclusively—solves the problem of non-uniform temperature acrossthe wafer under high RF heat loads. This is because heat transfer viathe latent heat of vaporization is a constant-temperature process. Inthe cooling mode of the PCHT loop, as it absorbs heat, the coolantinside the evaporator 200 does not change temperature. Instead, itchanges phase, going from liquid to vapor. Thus, all the coolantthroughout the evaporator 200 (the fluid passages inside the ESC base 5)is at a uniform temperature regardless of the magnitude of the RF heatload on the wafer. The advantage is that the wafer temperaturedistribution is about as uniform as the electric field distributionacross the ESC, so that the etch rate uniformity achieved under the mostfavorable conditions by the electrical features discussed earlier herein(e.g., the RF bias feedpoint impedance adjustment by multiple dielectricsleeves and the dielectric edge ring process kit) is maintained evenunder the highest RF heat loads, a result heretofore unattainable. Thisresult renders the reactor of FIGS. 1-7 useful for plasma processingunder the current design rules (small feature sizes) and for severalgenerations of future design rules in which feature sizes may shrinkeven further, a significant advantage. This advantage is combined withthe extremely high heat capacity of cooling through latent heat ofvaporization (discussed above), which provides about an order ofmagnitude greater heat flow rate than conventional (sensible) heattransfer via the coolant mass heat capacity.

Operation of the reactor of FIG. 7 in the foregoing manner that resultsin heat transfer through the coolant's latent heat of vaporizationcorresponds to the method illustrated in FIG. 9. The first step in thismethod is to enhance or optimize uniformity of radial distribution ofthe ESC temperature by maintaining the coolant that is inside theevaporator 200 within a range of temperatures and enthalpies at whichthe heat transfer is through contributions to (or deductions from) thecoolant's latent heat of vaporization. This step is depicted in block300 of FIG. 9. The step of block 300 may be carried out by limitingvariation in the orifice or opening size of the expansion valve 210 to arange which confines the temperature and enthalpy of the coolant in theevaporator 200 to lie inside the liquid-vapor dome 216 of thetemperature-enthalpy diagram of FIG. 8 (block 302 of FIG. 9). For agiven coolant and for a given coolant flow rate, the adjustment range ofthe expansion valve that confines the coolant inside the liquid-vapordome 216 of FIG. 8 is readily determined and can be pre-programmed intoa microprocessor controlling the entire system, for example. The step ofblock 300 may also be carried out by adjusting thecompressor-to-evaporator bypass flow valve 212 within a range in whichthe coolant inside the evaporator 200 is maintained inside theliquid-vapor dome 216 of FIG. 8 (block 304 of FIG. 9). The adjustment ofthe bypass valve 212 (in the step of block 304) and the adjustment ofthe expansion valve 210 (in the step of block 302) may be combined toachieve the desired result.

Once heat transfer through the latent heat of vaporization in theevaporator 200 has been established by the step of block 300, the nextstep is to control the ESC temperature (block 306 of FIG. 9). This maybe accomplished by adjusting the expansion valve 210 within the rangeestablished in the step of block 300 until a desired ESC temperature isreached (block 308 of FIG. 9). Alternatively, the ESC temperature may becontrolled by adjusting the compressor-to-evaporator bypass valve 212within the range established in the step of block 304. This latter stepcorresponds to block 310 of FIG. 9. Temperature control may also becarried out by performing the steps of blocks 308 and 310 together.

WORKING EXAMPLE

While the variable orifice size of the expansion valve 210 is theprimary control over cooling rate and wafer temperature, additional oralternative temperature control and, if desired, heating of the wafer,is provided by a compressor-to-evaporator bypass valve 212. Completeconversion of all liquid coolant to the gas phase in the accumulator 204can be ensured using a compressor-to-accumulator bypass valve 214.

While selection is readily made of a suitable coolant, a flow rate bythe compressor 206 and an orifice size of the expansion valve thatsatisfies the foregoing conditions, the following is provided as aworking example in which two-phase cooling is achieved:

ESC Inlet temperature: −10 to +50 deg C.

ESC Inlet pressure: 160 to 200 PSIG

ESC Inlet liquid-vapor ratio: 40%-60% liquid

ESC Inlet-Outlet max temperature difference: 5 deg C.

ESC Inlet-Outlet max pressure difference: 10 PSI

ESC Outlet Liquid-vapor ratio: 10% liquid

Accumulator outlet temperature: 60 to 80 deg C.

Accumulator outlet pressure: 25 to 35 PSIG

Accumulator outlet liquid-vapor ratio: 100% vapor

Compressor flow rate: 4 gal per min

Compressor outlet pressure: 260-270 PSIG

Compressor outlet temperature: 80-100 deg C.

Compressor outlet liquid-vapor ratio: 100% vapor

Condenser outlet temperature: 20-40 deg C.

Condenser outlet pressure: 250 PSIG

Condenser liquid-vapor ratio: 100% vapor

Expansion valve outlet liquid-vapor ratio: 80%

Some evaporation occurs between the expansion valve outlet and the ESCcoolant inlet 201, which explains the decrease in liquid-vapor ratiofrom 80% to 60% from the expansion valve 210 to the ESC inlet 201. Whileit may be preferable to constrain the thermal cycle within theliquid-vapor dome 216 of FIG. 8 (as discussed above), the invention maybe implemented with some excursion beyond that limit. In particular, thecoolant's liquid-vapor ratio may at least nearly reach zero at theevaporator outlet 202, or may reach zero just before the evaporatoroutlet 202, in which case a small amount of sensible heating may occur.In such a case, the vast majority of heat transfer still occurs throughthe latent heat of vaporization, only a small fraction occurring throughsensible heating, so that the advantages of the invention are realizednonetheless.

Large Range Temperature Feedback Control Loop:

Referring again to FIGS. 1 and 7, the wafer temperature may becontrolled or held at a desired temperature under a given RF heat loadon the wafer 110 using a temperature feedback control loop governingeither (or both) the expansion valve 210 and the bypass valve 212,although the simplest implementation controls the expansion valve 210only. The actual temperature is sensed at a temperature probe, which maybe a temperature probe 220 in the ESC insulating layer 10, a temperatureprobe 221 in the ESC base 05, a temperature probe 222 at the ESCevaporator inlet 201 or a temperature probe 223 at the ESC evaporatoroutlet 202 or a combination of any or all of these probes. For thispurpose, a feedback control loop processor 224 governs the orificeopening size of the expansion valve 210 in response to input or inputsfrom one or more of the temperature probes. The processor 224 isfurnished with a user-selected desired temperature value, which may bestored in a memory or user interface 225. As a simplified explanation,during each successive processing cycle, the processor 224 compares thecurrent temperature measured by at least one of the probes (e.g., by theprobe 220 in the ESC insulating layer) against the desired temperaturevalue. The processor 224 then computes an error value as the differencebetween the desired and measured temperature values, and determines fromthe error a correction to the orifice size of either the bypass valve212 or the expansion valve 210, that is likely to reduce the error. Theprocessor 224 then causes the valve orifice size to change in accordancewith the correction. This cycle is repeated during the entire durationof a wafer process to control the wafer temperature.

Agile Wafer Temperature Feedback Control Loop:

In conventional reactors, the wafer is cooled to avoid overheating fromabsorbed RF power by cooling the electrostatic chuck or wafer supportpedestal. Thermal conductivity between the wafer 110 and the cooled ESC105 is enhanced by injection under pressure of a thermally conductivegas (such as helium) into the interface between the backside of thewafer 110 and the top surface of the ESC 105, a technique well-known inthe art. For this purpose, gas channels 226 are formed in the topsurface of the ESC insulating layer 20 and a pressurized helium supply228 is coupled to the internal ESC gas channels 226 through a backsidegas pressure valve 229. The wafer 110 is electrostatically clamped downonto the top surface of the insulating layer 20 by a D.C. clampingvoltage applied by a clamp voltage source 128 to the grid electrode 15.The thermal conductivity between the wafer 110 and the ESC top layer 20is determined by the clamping voltage and by the thermally conductivegas (helium) pressure on the wafer backside. Highly agile (quick) wafertemperature control is carried out in accordance with the presentinvention by varying the backside gas pressure (by controlling the valve229) so as to adjust the wafer temperature to the desired level. As thebackside gas pressure is changed, the thermal conductivity between thewafer and the ESC top layer 20 is changed, which changes the balancebetween (a) the heat absorbed by the wafer 110 from RF power applied tothe grid electrode 15 or coupled to the plasma and (b) the heat drawnfrom the wafer to the cooled ESC. Changing this balance necessarilychanges the wafer temperature. A feedback control loop governing thebackside gas pressure can therefore be employed for agile or highlyresponsive control of the wafer temperature. The response of the wafertemperature to changes in the backside gas pressure is extremely quick(temperature changes reaching equilibrium within a second or less). Byway of comparison, changing the temperature of the base of the ESC orwafer support pedestal 105 does not cause the wafer to reach a new(elevated or depressed) equilibrium or steady state wafer temperaturefor on the order of minute (depending upon the thermal mass of the ESC105). Therefore, a temperature regulation system employing the backsidegas pressure provides agile temperature control capable of making fastadjustments to wafer temperature.

FIG. 7 illustrates such an agile temperature feedback control system, inwhich a feedback control loop processor 230 governs the backside gaspressure valve 229. One (or more) of the temperature sensors 220, 221,222 or 223 in the ESC may be connected to an input of the processor 230.A user interface or memory 231 may provide a user-selected or desiredtemperature to the processor 230. During each successive processingcycle, the processor 230 computes an error signal as the differencebetween the current temperature measurement (from one of the sensors220, 221, 222) and the desired temperature. The processor 230 determinesfrom that difference a correction to the current setting of the backsidegas pressure valve that would tend to reduce the temperature error, andchanges the valve opening in accordance with that correction. Forexample, a wafer temperature that is deviating above the desiredtemperature would require increasing the backside gas pressure toincrease thermal conductivity to the cooled ESC and bring down the wafertemperature. The converse is true in the case of a wafer temperaturedeviating below the desired temperature. The wafer temperature can thusbe controlled and set to new temperatures virtually instantly within atemperature range whose lower limit corresponds to the chilledtemperature of the ESC and whose upper limit is determined by the RFheat load on the wafer. For example, the wafer temperature cannot beincreased in the absence of an RF heat load and the wafer temperaturecannot be cooled below the temperature of the ESC. If this temperaturerange is sufficient, then any conventional technique may be used tomaintain the ESC at a desired chilled temperature to facilitate theagile temperature feedback control loop governing the backside gaspressure.

Dual Temperature Feedback Control Loops:

The agile temperature feedback control loop governing the backside gaspressure valve 229 and the large range temperature feedback control loopgoverning the refrigeration expansion valve 210 may be operatedsimultaneously in a cooperative combination under the control of amaster processor 232 controlling both feedback control loop processors224, 230.

The large range temperature feedback control loop (involving the PCHTloop consisting of the evaporator 200, the compressor 206, the condenser208 and the expansion valve 210) controls the workpiece temperature bychanging the temperature of the electrostatic chuck 105. The temperaturerange is limited only by the thermal capacity of the PCHT loop and cantherefore set the workpiece temperature to any temperature within a verylarge range (e.g., −10 deg C. to +150 deg C.). However, the rate atwhich it can effect a desired change in workpiece temperature at aparticular moment is limited by the thermal mass of the electrostaticchuck 105. This rate is so slow that, for example, with an electrostaticchuck for supporting a 300 mm workpiece or silicon wafer, a 10 degree C.change in workpiece temperature can require on the order of a minute ormore from the time the refrigeration unit begins to change the thermalconditions of the coolant to meet the new temperature until theworkpiece temperature finally reaches the new temperature.

In contrast, in making a desired change or correction in workpiecetemperature, the agile temperature feedback control loop does not changethe electrostatic chuck temperature (at least not directly) but merelychanges the thermal conductivity between the workpiece and theelectrostatic chuck. The rate at which the workpiece temperatureresponds to such a change is extremely high because it is limited onlyby the rate at which the backside gas pressure can be changed and thethermal mass of the workpiece. The backside gas pressure responds tomovement of the valve 229 in a small fraction of a second in a typicalsystem. For a typical 300 mm silicon wafer, the thermal mass is so lowthat the wafer (workpiece) temperature responds to changes in thebackside gas pressure within a matter of a few seconds or a fraction ofa second. Therefore, relative to the time scale over which the largerange temperature control loop effects changes in workpiece temperature,the workpiece temperature response of agile feedback loop iscomparatively instantaneous. However, the range over which the agilefeedback loop can change the workpiece temperature is quite limited: thehighest workpiece temperature that can be attained is limited by the RFheat load on the wafer, while the lowest temperature cannot be below thecurrent temperature of the electrostatic chuck 105. However, incombining the agile and large range temperature control loops together,the advantages of each one compensate for the limitations of the other,because their combination provides a large workpiece temperature rangeand a very fast response.

The master processor 232 may be programmed to effect large temperaturechanges using the large range feedback control loop (the processor 224)and effect quick but smaller temperature changes using the agilefeedback control loop (the processor 230). FIG. 10 is a graph of oneexample of wafer temperature behavior over time. The solid line depictsthe long term temperature behavior, in which the master processor 232effects slow large changes in wafer temperature using the large rangefeedback control loop with the processor 224. The dashed line depictsfast perturbations in temperature, in which the master processor 232effects fast but small changes in wafer temperature using the agilefeedback control loop with the processor 230.

The dual loop control afforded by the master processor 232 can beemployed to (nearly) instantly move the wafer temperature to a newdesired level and hold it there while the ESC temperature slowly changesto the new desired temperature. This is illustrated in FIGS. 11A and11B. The solid line in FIG. 11A depicts the wafer temperature behaviorover time in which the wafer temperature is stepped down to a lowertemperature at time t1 and held there, at which time the PCHT loop(dashed line) begins to cool down the ESC to the lower temperature,which is not reached by the ESC until time t2. The fast change in wafertemperature at time t1 and its temperature stability thereafter isaccomplished by the agile control loop 230. The agile control loopprocessor 230 receives the new (lower) desired wafer temperature at timet1 and responds by immediately increasing the backside gas pressure(FIG. 11B) to step the wafer temperature down to the new temperature attime t1. In the meantime, the ESC temperature begins to fall in order todrive the ESC to (or slightly below) the new temperature at time t1, sothat processor 224 increases the refrigeration cooling rate of the ESCto drive its temperature down. This forces the agile control loopprocessor 230 to decrease backside gas pressure after time t1 tomaintain the desired wafer temperature, until the ESC reaches thecorrect temperature at time t2, after which the backside gas pressureremains constant.

The example of FIGS. 12A and 12B illustrates how the ESC temperaturechange may be delayed while the PCHT loop is allowed to slowly adjust toa new temperature (to accommodate a time lag to the ESC surface of about50 degrees over 5 seconds). FIG. 12A depicts temperature behavior overtime while FIG. 12B depicts the corresponding backside gas pressureprofile over time. As illustrated in FIGS. 12A and 12B, the dual loopcontrol afforded by the master processor 232 can be employed totemporarily hold the wafer temperature constant (solid line of FIG. 12A)at an initial temperature level while, beginning at time t1, the PCHTloop takes the ESC through a large but slow temperature excursion(dashed line of FIG. 12A). Then, the wafer temperature is allowed tostep down to the new ESC temperature. This is accomplished by coolingthe ESC while constantly decreasing the backside gas pressure beginningat time t1. Then, after the desired ESC temperature is reached at timet2, the agile temperature control loop steps up the backside gaspressure to step the wafer temperature down to the ESC temperature.

Multiple Temperature Zones:

1. Large Range Temperature Control Loop:

The ESC 105 may be divided into plural radial zones, and differentindependent feedback control loops may separately control thetemperature in each zone. An advantage of this feature is that differentradial zones of the wafer 110 may be kept at different temperaturesduring processing so as to further reduce process or etch ratedistribution non-uniformities. In the example of FIG. 13, the ESC 105 isdivided into two temperature control zones, namely a radially inner zone234 and a radially outer zone 236, and a separate temperature controlapparatus is provided for each zone 234, 236. In some embodiments havingsuch plural radial zones, it may be preferable to divide the ESCconductive mesh or electrode 15 into plural radial zones (such asconcentric inner and outer zones 15 a, 15 b, for example).

The radially inner zone 234 of the aluminum base 05 contains inner zonecoolant flow passages 200 a with a coolant inlet 201 a and a coolantoutlet 202 a. The inner zone coolant flow passages 200 a constitute theinner zone evaporator of an inner zone PCHT loop, the evaporator 200 abeing internally contained with the inner zone 234 of the ESC base 05.The remaining elements of the inner zone PCHT loop are external of theESC 105, and include (in order of coolant flow direction, starting fromthe coolant outlet 202 a) an accumulator 204 a, a compressor 206 a, acondenser 208 a and an expansion valve 210 a having a variable orificesize, all of which are of the type well-known in the art. The radiallyouter zone 236 of the aluminum base 05 contains outer zone coolant flowpassages 200 b with a coolant inlet 201 b and a coolant outlet 202 b.The outer zone coolant flow passages 200 b constitute the outer zoneevaporator of an outer zone PCHT loop, the evaporator 200 b beinginternally contained with the outer zone 236 of the ESC base 05. Theremaining elements of the outer zone PCHT loop are external of the ESC105, and include (in order of coolant flow direction, starting from thecoolant outlet 202 b) an accumulator 204 b, a compressor 206 b, acondenser 208 b and an expansion valve 210 b having a variable orificesize, all of which are of the type well-known in the art. Temperature inthe inner zone 234 is sensed at one or more of the following inner zonetemperature probes: probe 220 a in the inner zone 234 of the ESCinsulating layer 10, probe 221 a in the inner zone of the ESC base 05,probe 222 a at the inner zone evaporator inlet 201 a or probe 223 a atthe inner zone evaporator outlet 202 a.

An inner zone feedback control loop processor 224 a governs the orificeopening size of the inner zone expansion valve 210 a in response toinput or inputs from one or more of the inner zone temperature probes.The inner zone processor 224 a is furnished with a user-selected desiredinner zone temperature value, which may be stored in a memory or userinterface 225 a. During each successive processing cycle, the inner zoneprocessor 224 a compares the current temperature measured by at leastone of the probes (e.g., the probe 220 a in the ESC insulating layer)against the desired temperature value and corrects the orifice size ofthe inner zone expansion valve 210 a accordingly. An outer zone feedbackcontrol loop processor 224 b governs the orifice opening size of theouter zone expansion valve 210 b in response to input or inputs from oneor more of the outer zone temperature probes. The outer zone processor224 b is furnished with a user-selected desired outer zone temperaturevalue, which may be stored in a memory or user interface 225 b. Duringeach successive processing cycle, the outer zone processor 224 bcompares the current temperature measured by at least one of the probes(e.g., the outer zone probe 220 b in the ESC insulating layer) againstthe desired temperature value and corrects the orifice size of the outerzone expansion valve 210 b accordingly.

2. Agile Temperature Feedback Control Loop:

In both temperature zones 234 and 236, thermal conductivity between thewafer 110 and the cooled ESC 105 is enhanced by injection under pressureof a thermally conductive gas (such as helium) into the interfacebetween the backside of the wafer 110 and the top surface of the ESC105, a technique well-known in the art. In the inner temperature zone234, inner zone gas channels 226 a are formed in inner zone 234 of thetop surface of the ESC insulating layer 20 and a pressurized heliumsupply 228 a is coupled to the inner zone gas channels 226 a through aninner zone backside gas pressure valve 229 a. The wafer 110 iselectrostatically clamped down onto the top surface of the insulatinglayer 20 by a D.C. clamping voltage applied by a clamp voltage source128 to the grid electrode 15 (i.e., 15 a and 15 b). The thermalconductivity between the wafer 110 and the ESC top layer 20 isdetermined by the clamping voltage and by the thermally conductive gas(helium) pressure on the wafer backside. Highly agile (quick) wafertemperature control is carried out in the inner temperature zone 234 bycontrolling the inner zone valve 229 a so as to adjust the wafertemperature to the desired level. An inner zone agile feedback controlloop processor 230 a governs the inner zone backside gas pressure valve229 a. One (or more) of the inner zone temperature sensors 220 a, 221 a,222 a or 223 a in the ESC inner zone 234 may be connected to an input ofthe inner zone agile processor 230 a. An inner zone user interface ormemory 231 a may provide a user-selected or desired temperature to theinner zone agile processor 230 a. During each successive processingcycle, the processor 230 a senses an error as the difference between thecurrent temperature measurement (from one of the inner zone sensors 220a, 221 a, 222 a) and the desired temperature, and changes the opening ofthe inner zone backside gas valve 229 a accordingly.

In the outer temperature zone 236, outer zone gas channels 226 b areformed in outer zone 236 of the top surface of the ESC insulating layer20 and the pressurized helium supply 228 b is coupled to the outer zonegas channels 226 b through an outer zone backside gas pressure valve 229b. Highly agile (quick) wafer temperature control is carried out in theouter temperature zone 236 by controlling the outer zone valve 229 b soas to adjust the wafer temperature to the desired level. An outer zoneagile feedback control loop processor 230 b governs the outer zonebackside gas pressure valve 229 b. One (or more) of the outer zonetemperature sensors 220 b, 221 b, 222 b or 223 b in the ESC outer zone236 may be connected to an input of the outer zone agile processor 230b. An outer zone user interface or memory 231 b may provide auser-selected or desired temperature to the inner zone agile processor230 b. During each successive processing cycle, the processor 230 bsenses an error as the difference between the current temperaturemeasurement (from one of the outer zone sensors 220 b, 221 b, 222 b) andthe desired temperature, and changes the opening of the outer zonebackside gas valve 229 b accordingly.

With the combination of the agile and large range inner and outerfeedback control loops described above with reference to FIG. 13, theradial profile of the wafer temperature may be controlled over a largerange with agile response.

Temperature Probe with Minimal or No RF Parasitics:

FIG. 14 depicts a preferred temperature probe 238 installed in theplasma reactor of FIG. 1. The probe 238 consists of two separableportions, namely an upper probe 239 installed in the ESC 105 and a lowerprobe 240 installed in a portion of the reactor chamber beneath andsupporting the ESC 105, namely a chamber host base 241. The upper probe239 is depicted in the enlarged view of FIG. 15, and lies in an area ofhigh RF electric potential (i.e., inside the ESC insulating layer orpuck 10, 20). The upper probe 239 is firmly inserted in an elongateaxial hole within the ESC 105 that closely fits the upper probe 239, andthe tip of the upper probe 239 lies very close (e.g., within 3 to 3.5mm) to the top surface of the puck 20. (The advantage is that the probe239 is sufficiently close to the wafer 110 to minimize or eliminatetemperature measurement errors.) This area of the ESC has very highelectric field potential during processing so that any electricalproperties that the upper probe 239 may have would have profound effectson plasma processing on the wafer. The upper probe 239 thereforeincludes RF compatibility features which minimize or eliminate anyeffect that the probe 239 might otherwise have on the electric field oron the RF impedance distribution. Such RF compatibility features ensurethat the probe 239 does not distort or perturb the ESC electric field orRF impedance distribution that has been so carefully adjusted with thefeatures of the feedpoint impedance adjustment of FIGS. 2-4 and/or thedielectric ring process kit of FIGS. 5-6 (for example). The RFcompatibility features of the upper probe 239 include a complete absenceof any conductive materials within the probe 239, an orientation of theprobe in the axial direction (to minimize its effect on the radialelectric field or RF impedance distribution) and its small diameter,which is on the order of a fraction of a Debeye length of the plasma inthe chamber. These features are made possible by employing anelectrically nonconductive optical temperature transducer 242 (e.g.,phosphor material) whose blackbody radiation spectrum is a well-knownfunction of its temperature. The optical temperature transducer 242 iscoupled to a long thin optical fiber 243 contained within the thin axialupper probe 239. The upper probe 239 further includes an opaquecylindrical dielectric sleeve 244 surrounding the optical fiber 243 andpreferably consisting of glass-impregnated plastic. The opticaltemperature transducer 242 is capped by a dielectric cap 245 of amaterial that is, preferably, identical to the dielectric material ofthe ESC puck 10, 20, which in the preferred embodiment is aluminumnitride. This latter feature ensures that the temperature behavior ofthe material contacting the optical temperature transducer 242 (i.e.,the cap 245) is identical to the material whose temperature is to bemeasured (i.e., the ESC puck layer 20 that is in direct contact with thewafer 110).

The upper probe 239 further includes a mounting plate 246 that isremovably fastened to the bottom surface of the ESC base 05. Themounting plate 246 supports a spring housing 247 containing a coilspring 248 compressed between a shoulder 245 of the housing 247 and anannular ring 249 fastened to a portion of the probe sleeve 244 lyingwithin the housing 247. As the upper probe 239 is inserted into the ESC105 and presses against the top end of the hole within the ESC, the coilspring 248 is compressed to force the tip of the probe 239 to self-alignto the top end of the hole.

The lower probe 240 is shown in the enlarged view of FIG. 16 andincludes an optical fiber 250 surrounded by an opaque lower cylindricalsleeve 251. Since the lower probe 240 is below the grounded conductiveESC base 05, it is located outside of areas of high RF electric fields,and therefore need not be formed of non-conductive materials. In fact,the lower cylindrical sleeve 251 may be formed of steel, for example.The top end 252 of the lower probe 240 is tightly received within a hole253 in the mounting plate 246 of the upper probe 239. The lower probe240 further includes a mounting plate 254 that is removably fastened tothe bottom surface of the chamber housing host base 241. The mountingplate 254 supports a spring housing 255 containing a coil spring 256compressed between a shoulder 257 of the housing 255 and an annular ring258 fastened to a portion of the lower probe sleeve 251 lying within thehousing 255. As the tip 252 of the lower probe 240 is inserted into thehole 253 of the upper probe mounting plate 246 and pressed against thetop end of the hole 253, the coil spring 256 is compressed to force thetip of the lower probe 240 to self-align to the top end of the hole 253.The resulting self-alignment of the lower probe 240 against the upperprobe 239 is illustrated in FIG. 17, which shows that the facing ends ofthe upper probe optical fiber 243 and the lower probe optical fiber 250are in nearly perfect alignment. Signal conditioning circuitry convertsthe light received from the optical fiber at the bottom end of the lowerprobe fiber 250 and converts it to a digital signal for use by one ofthe feedback control loop processors. While FIG. 14 depicts a singletemperature probe whose tip lies near the top of the ESC 105, anotheridentical probe may be placed in a lower portion of the ESC but at thesame radial location as first probe. Other identical probes may beplaced at different radial (azimuthal) locations within the ESC but inthe same height (axial location) as other probes. Thus, the temperatureprobes 220 a, 220 b of the different temperature zones 234, 236 of FIG.13 may each be of the type described above in FIGS. 13-16 and arelocated at different radial locations at a common axial height.

While certain embodiments of the invention have been described asincluding different feedback control loop processors, any or all suchprocessors may be implemented in a single, common processor programmedto perform the functions of each of the individual feedback control loopprocessors. Similarly, other resources associated with the differentcontrol loops, such as the dual helium supplies 228 a, 228 b, may beimplemented with a single supply or resource with separately controlledinterfaces (e.g., such as a single helium supply and dual pressurecontrol valves 229 a, 229 b). Moreover, if (for example) the conductivemesh electrode 15 is divided into inner and outer electrodes 15 a, 15 bas suggested earlier in this specification, then a common RF bias powersource may be employed to apply different levels of RF bias power to theinner and outer mesh electrodes 15 a, 15 b. Alternatively, separate RFbias power generators may be employed to realize the separate RF biaspower levels.

While the invention has been described in detail by specific referenceto preferred embodiments, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention.

1. A method of transferring heat from or to a workpiece support in an RFcoupled plasma reactor, comprising: placing coolant in an internal flowchannel that is located inside said workpiece support; transferring heatfrom or to said coolant by circulating said coolant through arefrigeration loop in which said internal flow channel of said workpiecesupport constitutes an evaporator of the refrigeration loop; maintainingthermal conditions of the coolant inside said evaporator within a rangein which heat exchange between said workpiece support and said coolantis primarily or exclusively through the latent heat of vaporization ofsaid coolant.
 2. The method of claim 1 wherein the step of maintainingthermal conditions prevents or minimizes temperature changes in thecoolant inside said evaporator, whereby to maintain an at least nearlyuniform temperature distribution across said workpiece support.
 3. Themethod of claim 1 wherein the step of maintaining thermal conditionscomprises maintaining the temperature and enthalpy of the coolant withinsaid evaporator within a range in which said coolant exists in both itsvapor and liquid phases simultaneously.
 4. The method of claim 1 whereinthe step of maintaining thermal conditions creates a difference in theliquid-to-vapor ratio of said coolant across said evaporator inaccordance with the heat flow rate between said workpiece support andsaid coolant.
 5. The method of claim 4 wherein the liquid-to-vapor ratioof coolant near the inlet of said evaporator is between about 40% and80% while the liquid-to-vapor ratio of coolant near the outlet of saidevaporator does not exceed about 10%.
 6. The method of claim 4 whereinthe liquid-to-vapor ratio of the coolant upon exiting said evaporator isminimal or nearly zero.
 7. The method of claim 1 wherein saidrefrigeration loop comprises, in the order of flow of said coolantthrough said refrigeration loop, said evaporator, an accumulator, acompressor, a condenser and an expansion valve and acompressor-to-evaporator bypass channel governed by a bypass valve, andwherein the step of maintaining the thermal conditions comprises:controlling said expansion valve and said bypass valve so as to achieveheat transfer through the latent heat of vaporization of said coolant.8. The method of claim 7 further comprising: controlling the temperatureof said wafer support in accordance with a desired temperature value bycontrolling said expansion valve.
 9. The method of claim 8 wherein thestep of controlling the temperature of the workpiece support comprises:receiving a measured temperature of the workpiece support or of aworkpiece on the workpiece support; and adjusting an opening size ofsaid expansion valve so as to reduce the difference between saidmeasured temperature and said desired temperature value.
 10. In an RFcoupled plasma reactor, a method of transferring heat from or to aworkpiece support with an at least nearly uniform temperature across theworkpiece support, comprising: placing coolant in an internal flowchannel that is located inside said workpiece support; transferring heatfrom or to said coolant by circulating said coolant through arefrigeration loop in which said internal flow channel of said workpiecesupport constitutes an evaporator of the refrigeration loop; maintainingan at least nearly uniform temperature of the coolant throughout saidevaporator during the step of transferring heat from or to the coolantin said evaporator.
 11. The method of claim 10 wherein the step oftransferring heat comprises removing heat from said coolant at asufficient rate to compensate for RF heat loading of a workpiece on saidworkpiece support.
 12. The method of claim 11 wherein the step ofmaintaining an at least nearly uniform temperature comprises limiting atemperature difference across a top surface of said workpiece support toless than a tenth of the temperature of said workpiece support topsurface.
 13. The method of claim 12 wherein said temperature differencedoes not exceed about 5 degrees C.
 14. The method of claim 10 whereinthe step of maintaining an at least nearly uniform temperature comprisesmaintaining coolant within said evaporator within a mixed liquid andvapor phase.
 15. The method of claim 14 wherein the step of maintainingsaid mixed liquid and vapor phase creates a difference in theliquid-to-vapor ratio of said coolant across said evaporator inaccordance with the heat flow rate between said workpiece support andsaid coolant.
 16. The method of claim 10 wherein said refrigeration loopcomprises, in the order of flow of said coolant through saidrefrigeration loop, said evaporator, an accumulator, a compressor, acondenser and an expansion valve and a compressor-to-evaporator bypasschannel governed by a bypass valve, and wherein the step of maintainingthe thermal conditions comprises: controlling said expansion valve andsaid bypass valve so as to achieve heat transfer through the latent heatof vaporization of said coolant.
 17. The method of claim 16 furthercomprising: controlling the temperature of said wafer support inaccordance with a desired temperature value by controlling saidexpansion valve.
 18. The method of claim 17 wherein the step ofcontrolling the temperature of the workpiece support comprises:receiving a measured temperature of the workpiece support or of aworkpiece on the workpiece support; and adjusting an opening size ofsaid expansion valve so as to reduce the difference between saidmeasured temperature and said desired temperature value.